Myocardin-related Transcription Factors Control Epicardial EMT and Coronary Vessel Homeostasis

The epicardium is a source of multi-potent progenitor cells that envelop the heart and contribute to various cardiac lineages through the process of epithelial-to-mesenchymal transition (EMT). Although known upstream cues promote epicardial EMT via changes in gene expression and actin cytoskeletal dynamics, the link between these signals and progenitor cell motility remain unclear. Myocardin-related transcription factor (MRTF) cofactors are primarily retained in the cytoplasm through interactions with G-actin. However, upon cytoskeletal reorganization and G-actin depletion, nuclear MRTFs associate with serum response factor (SRF) to drive cellular motility and contractility gene programs reminiscent of EMT. Here, we identify the SRF/MRTF gene regulatory axis as a key mediator of epicardial EMT. We found significant enrichment of MRTF-A and -B in the epicardium prior to EMT. MRTFs were later expressed in a spatial and temporal manner concurrent with EMT and epicardial-derived cell (EPDC) differentiation. Furthermore, MRTF deletion attenuates contractile gene expression in epicardial explants and impairs migration of EPDCs into subjacent cell layers using ex vivo assays. Epicardial EMT and EPDCs differentiation are necessary for proper coronary vessel formation. We found that genetic ablation of MRTFs results in disrupted coronary plexus formation, endothelial cell dysfunction, and sub-epicardial hemorrhage. The vascular phenotype observed in MRTF-A/B epiDKO mice results in part from the depletion (~50% reduction) of epicardial-derived coronary pericytes. These data suggest a critical role for MRTFs in coronary vessel formation by regulating epicardial EMT and mobilizing EPDCs.